The arrest, days after the letters were intercepted in mail-sorting facilities for the White House and the Capitol, was based on information collected “very early on” about who had sent the letters, an official said.

Corinth is about 50 miles north of Tupelo, the hometown of Roger Wicker, the Republican senator to whom one of the letters was addressed. The letters, which had a Memphis postmark but no return address, were signed, “I am KC and I approve this message,” according to Senate staff members briefed on the case. Corinth is about 100 miles east of Memphis.

The speedy arrest in the case may calm nerves in a twitchy capital where it had begun to feel like the weeks after the Sept. 11, 2001, terrorist attacks, when anthrax-laced letters mailed to media organizations and two Democratic senators killed 5 people and sickened 17 others.

Photo

A firefighter at a mail screening facility in Hyattsville, Md.Credit
Alex Brandon/Associated Press

Earlier in the day, with reports of other suspicious letters and packages turning up in Senate offices, the Capitol Police evacuated parts of the Hart and Russell Senate Office Buildings, shouting to clear the hallways and leaving staff members to peer nervously out their windows.

At the White House, where yellow police tape had been stretched across Pennsylvania Avenue since Monday afternoon to keep pedestrians off the street in front of the residence, the press secretary, Jay Carney, urged patience for those investigating the Boston bombings, discouraged efforts to link the attack with the contaminated letters and generally appealed for calm.

“You heard the president say that we will not be terrorized,” Mr. Carney said. “And I think you saw and have seen, in the reaction of the people of Boston and in the reaction of Americans across the country, the proof of that statement. And that’s who we are.”

Though the F.B.I. said in a statement “there is no indication of a connection to the attack in Boston,” Mr. Carney, at a briefing in the White House, did not rule it out.

To be sure, there was less of the ambient dread that followed the 2001 attacks, when the letters laced with anthrax were sent. Those letters set off widespread panic, with fears that Al Qaeda was behind them. The White House ordered staff members to take an antibiotic, Cipro, as a protective measure. The F.B.I., after a long investigation, concluded that a military scientist, Bruce Ivins, had sent the letters. He was about to be charged in 2008 when he committed suicide.

In response to that episode, all mail addressed to the White House and the Capitol is now screened at outside sorting facilities, where the letters to Mr. Obama and Mr. Wicker were intercepted. Although they showed evidence of ricin in a preliminary test, the F.B.I. cautioned that such tests are often faulty and said further testing was needed for a conclusive result.

“Only a full analysis performed at an accredited laboratory can determine the presence of a biological agent such as ricin,” the F.B.I. said in a statement. “Those tests are currently being conducted and generally take 24 to 48 hours.”

Leonard A. Cole, an expert in bioterrorism who teaches at the University of Medicine and Dentistry of New Jersey, urged caution. “We’re used to false positives,” he said. “I have no doubt that if there was no Boston Marathon bombing, we wouldn’t even be talking about the ricin letters right now.”

But Dr. Cole, who has written a book called “The Anthrax Letters,” said the parallels to that episode were unmistakable. People tend to seize on the publicity generated by terrorist attacks, he said, to perpetrate copycat attacks — some genuine, more of them hoaxes.

Ricin, if inhaled or ingested, can be every bit as lethal as anthrax. A byproduct of the castor oil plant, it is highly toxic. If converted into a powder, a few grains of it can be deadly, and there is no antidote. It was used most famously in 1978, when an assassin killed a Bulgarian dissident, Georgi Markov, on a London street by firing into his leg a tiny ricin-laced pellet from a modified umbrella.

Video

Ricin: Poison of Choice Over the Years

This toxic chemical comes from the bright red castor plant and can be deadly if inhaled.

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The F.B.I. said it arrested Mr. Curtis at 5:15 p.m. at his home in Corinth. Mr. Curtis is suspected of sending a third contaminated letter to a Mississippi justice official, a person briefed on the case said.

On Capitol Hill, the tension remained palpable. At midday, the Capitol Police confirmed that a suspicious package was found on the atrium level of the Hart building. Officers questioned a man about the suspicious packages but reopened the building a short time later.

Senator Carl Levin, Democrat of Michigan, also released a statement on Wednesday, saying a suspicious letter had been found at his office in Saginaw. “The letter was not opened, and the staffer followed the proper protocols for the situation, including alerting the authorities, who are now investigating,” he said.

Asked about the police tape at the White House, Mr. Carney said he did not know of a specific threat. But he said the Secret Service sometimes extended the security perimeter “out of an abundance of caution.”

Ashley Parker contributed reporting.

A version of this article appears in print on April 18, 2013, on Page A17 of the New York edition with the headline: Arrest Is Made for Letters Believed to Have Poison. Order Reprints|Today's Paper|Subscribe